1. Field of the Invention
The present invention concerns a digital radiotelephone installation with mobile terminals known as "earphones". It concerns in particular mobile terminals that can be used in an installation of this kind and specific access control means included in the latter.
2. Description of the Prior Art
In a conventional digital mobile radiotelephone installation, the radio coverage is currently organized in the following manner:
The terrestrial coverage area is divided into a mosaic of hexagonal cells having a radius between one kilometer and 35 kilometers, depending on the environment. In open country it is in the order of 35 kilometers whereas in urban areas it is in the order of 1 kilometer to 3 kilometers.
A fixed base transceiver station (BTS) is provided at the center of each hexagonal coverage cell and provides the radio link to all the mobile terminals within the perimeter of that cell. It therefore comprises all the radio equipment necessary to provide the coverage of a cell.
Each digital mobile terminal is usually adapted to receive a plug-in smart card ("SIM card") that contains all the data specific to the user's subscription and enables the latter to access the telephone network via the nearest base transceiver station. Insertion of the SIM card by the user therefore personalizes the terminal and renders it operational. Access to the network is usually possible only after entering the personal access code ("PIN code") using the keypad of the mobile terminal.
To this end the network includes access control means constituting what are usually called a Home Location Register (HLR) and a Visitor Location Register (VLR) holding such data and codes specific to the user's subscription and adapted selectively to enable calls involving that user. The BTS consult the HLR and the VLR.
However, many users require to be contactable at all times, even when they are absent. Consequently, a number of manufacturers additionally offer on each mobile terminal an answering function and/or a short messages service (SMS) function.
A mobile terminal is designed to be inoperative if the SIM card is not inserted into it, with the exception of the ability to make a number of emergency calls, of course; the user must therefore leave their SIM card in the mobile terminal if they want to use the telephone answering machine function and/or the short messages service function. The user must also have entered their access code, of course.
This conventional solution has two major drawbacks:
the user is then unable, when moving around, to use their SIM card to phone from another terminal, since the card must remain in the original terminal if messages are to be received; PA1 the mobile terminal is not secure, since it contains an SIM card for which the access code has been entered:
anyone can then telephone using the terminal, with the user responsible for the charges incurred, and if the terminal is stolen it can immediately be used to make calls, again at the cost of the user.
The article "Une application de la carte a microprocesseur: le module d'identite d'abonne du radiotelephone numerique europeen" ("A smart card application: the subscriber identity module of the European digital radiotelephone") by P. Jolie et al published in l'Echo des Recherches, N.degree. 139, 1st quarter 1990, mentions a digital radiotelephone mobile terminal adapted to receive an extractable personalized smart card (SIM card) and another, auxiliary SIM smart card that remains in the terminal at all times, this auxiliary SIM card corresponding to the same personalized subscription as said conventional SIM card and having the same functionality as the latter, although the extractable conventional SIM card takes precedence over said auxiliary SIM card when it is inserted into the mobile terminal.
However, as both cards have the same functions the problem is exactly as previously stated.
A terminal using two SIM cards is also described in document EP-0 556 970A1 but this has only one card reader and uses only one card at a time, the other one remaining accessible, which does not solve the problem as stated here; neither does document WO 91/12 698 which describes a set with only one card.